The invention relates to a developing unit which may be used in an electrostatic copying machine of the wet type, for example, and more particularly to a developing unit of the type including a developing electrode which has applied thereto a bias potential corresponding to the potential of the background of an electrostatic latent image.
As is well recognized, in an electrophotographic arrangement employing a photosensitive member which is formed of an organic, photoconductive semiconductor, wear of the photosensitive member may cause a variation in the residual potential on the background portion of the electrostatic latent image which corresponds to the background portion of an original being copied, thereby resulting in a background smearing. The background smearing may also be caused by the influence of the optical system or the developing solution used. A developing electrode is generally disposed in opposite relationship with the photosensitive member and has applied thereto a bias voltage which is usually of a constant magnitude. This reduces the biasing effect when a variation occurs in the residual potential of the background of the image as mentioned above.
To overcome this difficulty, there has previously been proposed (Japanese Patent Application No. 67714/1974) an automatic control technique to prevent the background smearing, in which a detecting electrode is located adjacent to that one end of the developing electrode which is first to be opposed by the photosensitive member, for detecting the potential of the background of the latent image in order to automatically establish a bias potential, applied to the developing electrode, which varies in accordance with the detected potential, thus enabling a high quality visual image to be obtained which is free from the background smearing.
When detecting the surface potential of the photosensitive member for the purpose mentioned, it is necessary to detect an electric field of an extremely low magnitude in the space surrounding the photosensitive member, so that a chopper circuit having a high gain must be provided, with the consequence that the detecting device becomes an expensive unit. The high gain is required of the detecting circuit because of the arrangement of the detecting electrode, which is either connected with the ground, maintained at a suitable potential, or secured to a shielding case through an insulating bracket.
The automatic control technique mentioned above is characterized by the detection of the surface potential within a developing solution having a relatively high conductivity in order to avoid the disadvantage mentioned in the immediately preceding paragraph. However, a mere provision of the detecting electrode in the solution is susceptible to the influence of the potential of the image region or other current noises, resulting in the generation of a large error.